1. Field of the Invention
The present invention related to an electrophotographic photoreceptor and an image forming apparatus.
2. Description of the Related Art
Electrophotographic image forming apparatus have been utilized not only for copying machines but also generally for printers as output means of computers, etc. for which demand has been remarkably increased in recent years. In electrophotographic image forming apparatus, a photosensitive layer of an electrophotographic photoreceptor provided to the apparatus is uniformly charged by a charger, exposing the same, for example, by a laser light corresponding to image information, and a finely particulate developer, which is called as a toner, is supplied to electrostatic latent images formed by exposure from a developing device, to form toner images.
Although toner images formed by adhesion of the toner as a component of a developer to a surface of the electrophotographic photoreceptor is transferred to a transfer material such as recording paper by transfer means, not all the toner on the surface of the electrophotographic photoreceptor is transferred to the recording paper but the toner partially remains on the surface of the electrophotographic photoreceptor. Further, paper dusts of recording paper in contact with the electrophotographic photoreceptor during development may sometimes remain being deposited to the electrophotographic photoreceptor as they are.
Since the residual toner and deposited paper dusts on the surface of the electrophotographic photoreceptor give adverse effects on quality of images to be formed, the residual toner and deposited paper dusts are removed by a cleaning device. Further, a cleanerless technique has been developed in recent years in which the residual toner and deposited paper dusts are removed by a so-called development and cleaning system in which the residual toner is recovered by a cleaning function added to the developing means without providing independent cleaning means. To the electrophotographic photoreceptor, since operations of charging, exposure, development, transfer, cleaning and charge elimination are conducted repetitively, resistance against electrical and mechanical factors have been demanded. Specifically, it has been required for wear resistance against abrasion or scratches occurred upon frictional rubbing to the surface of the electrophotographic photoreceptor or durability against degradation of the surface layer caused by deposition of active substances such as ozone or NOx generated upon charging by the charger.
For attaining cost reduction and maintenance free with respect to the electrophotographic image forming apparatus, it is important that the electrophotographic photoreceptor has sufficient wear resistance and durability and can operate stably for a long period of time. Physical properties of the surface layer constituting the electrophotographic photoreceptor are greatly concerned with the wear resistance, the durability and the long time stability of operation of the electrophotographic photoreceptor. Heretofore, the electrophotographic photoreceptor has been designed to improve the durability by increasing the ratio of a polymeric binder used for the surface layer or by using a binder of a large molecular weight. However, increase of the binder ratio decreases the sensitivity of the photoreceptor and this is not suitable to high speed operation. Further, a binder of large molecular weight involves a problem of increasing the viscosity of a coating solution and thus leads to poor productivity. In view of the foregoings, it has been demanded for making the photoreceptor highly resistant to printing by a quantitative evaluation method.
Hardness is one of indices that evaluate not only physical properties on the surface of an electrophotographic photoreceptor but also generally physical properties of the materials, particularly, mechanical properties. The hardness is defined as a stress from a material against indentation urging of an indenter. An attempt of quantitizing mechanical properties of a film constituting the surface of the electrophotographic photoreceptor by using the hardness as a physical parameter for recognizing physical properties of materials has been conducted. For example, scratch test, pencil hardness test and Vickers hardness test, etc. have been generally known as a test method for measuring the hardness.
However, any of the hardness tests described above involves a problem in measuring mechanical properties of a material sowing complicate behaviors of plasticity, elasticity (also including retarded component) and creeping property in combination. For example, while Vicker's hardness is used for the evaluation of hardness of a film by measuring the length of an indentation, this reflects only the plasticity of the film and can not exactly evaluate a mechanical property showing a deformation state also including a large rate of elastic deformation such as an organic material. Accordingly, the mechanical property of a film constituted with an organic material has to be evaluated while considering various properties.
In an electrophotographic photoreceptor having an organic photosensitive layer at the surface layer, plastic deformation energy ratio (plastic deformation ratio ηplast %), elastic work efficiency (elastic deformation ratio ηHU %) etc. have been proposed as the physical property for judging the wear resistance, the durability and the operation stability for a long time of an organic photosensitive layer (refer, for example, to Japanese Unexamined Patent Publications JP-A 2000-10320 and 2002-6526). The plastic deformation energy is a ratio of the plastic deformation energy relative to the sum for a plastic deformation energy (energy required for plastic deformation) and elastic deformation energy (energy required for elastic deformation) represented by percentage. Further, the elastic work efficiency is a ratio of the elastic deformation work energy relative to the sum for the plastic deformation energy and the elastic deformation work energy by the percentage. Accordingly, the sum for plastic deformation energy ratio and the elastic work efficiency is 100(%).
More specifically, JP-A 2000-10320 proposes to set the plastic deformation energy ratio (plastic deformation ratio) to 30 to 70% and set a universal hardness value by universal hardness test according to DIN50359-1 (Hu) to 230 to 700 N/mm2. JP-A 2000-10320 describes that mechanical deterioration for the photoreceptor surface layer is prevented by setting such a range for the numerical values. However, the range for numeral values of the plastic deformation energy of 30 to 70% is a range including substantially all of organic photosensitive layers containing binder resins used generally at present. Accordingly, even when the plastic deformation energy ratio is within the range described above, this can not always provide an organic photosensitive layer excellent in long time wear resistance, durability and operation stability.
Further, JP-A No. 2002-6526 proposes an electrophotographic photoreceptor having an organic photosensitive layer and a protective layer containing a curable resin as a binder resin on a conductive substrate, and in which the elastic work efficiency ηHU of the protective layer (=[elastic deformation energy/(plastic work energy+elastic deformation energy)]×100) is from 32 to 60%. However, the numerical values of 32 to 60% for the elastic work efficiency is identical with that of 40 to 68% for the plastic deformation energy ratio which is a range including substantially all of electrophotographic photoreceptors formed with organic photosensitive layers as the surface layer. Further, the curable resin used as the binder resin is also ordinary in the technical field of the electrophotographic photoreceptor. Accordingly, JP-A 2002-6526 neither discloses means for solution in order substantially to obtain an organic photosensitive layer excellent in the long time wear resistance, durability, and operation stability. Further, the electrophotographic photoreceptor of JP-A 2002-6526 involves a problem of increasing the cost in the formation of the protective layer containing the curable resin.